As a cam motor apparatus of such kind, there is known a conventional cam motor apparatus configured such that a plurality of pistons and cylinders are divided into four groups and the condition of distribution of working oil to the pistons and cylinders included in each group is changed between two stages through the operation of a selector valve (See, for example, FIG. 2 in Japanese Patent Application Laid-Open Gazette No. 55-153871). In this motor, when the selector valve is changed to a low-speed mode, working oil is supplied to each cylinder included in two groups selected from among the four groups, whereas each cylinder included in the other two groups is connected to an oil tank so as to discharge working oil thereto. Thereby, the motor capacity of the cam motor apparatus becomes a maximum value so that the motor is rotated at relatively low speed and high output torque.
On the other hand, when the selector valve is changed to a high-speed mode, working oil is supplied to each cylinder included in one of the selected two groups, working oil is discharged from each cylinder included in one of the other two groups, and cylinders included in the remaining two groups are connected to each other to form a closed circuit. Thereby, the motor capacity becomes half the value in the low-speed mode so that high-speed rotation is made at a speed approximately twice as fast as the low-speed mode.
In the conventional cam motor apparatus above-described, however, since each cylinder included in the two groups in which neither oil supply nor oil discharge is made in the high-speed mode has a closed circuit, no escape route is left for pressurized oil in the cylinder so that large resistance to the motor rotation may be produced. To cope with this, it can be considered that each cylinder included in the above-mentioned two groups is communicated with the oil tank. In this case, however, the oil pressure of each cylinder included in the two groups becomes close to the atmospheric pressure so that slide contact between the piston and the cam surface in the cylinder cannot be held by the oil pressure. As a result, undesirable beat sounds are produced due to collision between the piston and the cam surface, and the piston and the cam surface are decreased in durability.
To eliminate the above problems, it is necessary to dispose a spring between the piston and the bottom surface of the cylinder room as in the conventional cam motor apparatus so as to press the piston against the cam surface by the spring. In this case, however, the component count of the apparatus is increased. This increases the weight of the apparatus and complicates the structure thereby involving much expense in time and effort for assembly.
In view of the foregoing problems, the present invention has been made. Therefore, an object of the present invention is to hold slide contact between the piston and the cam surface thereby increasing silentness and durability and to decrease component count thereby reducing weight and increasing ease of assembly.